What mileage does your slant six powered ride get?

Slant 6 Engines

  1. cruiser

    cruiser Well-Known Member

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    Yeah, I know. It's been asked a million times, but here goes once again. I'm wondering if I should get better mileage in my extremely stock 74 Duster with the 225 auto and Holley 1945 stock carb. Timing is factory 0.0 TDC. Everything stock under the hood and in the drive train. I get 14.5 in the city and 16.5 on the highway. Is this normal? Or too low? BTW, I put the car on a chassis dyno and got 79 rear wheel horsepower form this 50K mile motor. Thoughts?
     
  2. toolmanmike

    toolmanmike FABO Staff Staff Member FABO Gold Member

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    Not a slant guy but 5-10° before should get you better mileage.
     
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    • toolmanmike

      toolmanmike FABO Staff Staff Member FABO Gold Member

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      I have a friend with a stock slant in a early 40's Dodge coupe with a 4:10 rear gear, tall tires and a o/d 4 speed that gets 26. Strange combo I know.
       
    • cruiser

      cruiser Well-Known Member

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      Would that harm the engine of create a loss of power?
       
    • toolmanmike

      toolmanmike FABO Staff Staff Member FABO Gold Member

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      Not enough timing makes it a dog. Too much timing causes detonation (pinging) andcan cause damage.
       
    • dibbons

      dibbons FABO Gold Member FABO Gold Member

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      My vacuum advance failed right before a road trip one time and that really put a dent in gas mileage. That was a 318 BBody, which now gets about 17 MPG on the freeway.
       
    • jas0162

      jas0162 Well-Known Member

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      Make sure your vacuum advance is working properly and put in as much timing as you can without detonation. Get an a/f ratio gauge if you really want to dial in the carb. I messed with a slant in my dd for a while and got around 16 most of the time, put in a stock 73 318 and now I get 18 with the same 2bbl carb and ignition system I had on the slant.
       
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      • rumblefish360

        rumblefish360 So close, yet so far away FABO Gold Member

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        Not to crap on your parade but that’s with an overdrive and the final ratio is what?
        2.82 w/a .69 OD ratio? No wait, manual, it’s higher right?

        Where are the guys that claim 30, 35, 37+ mpg’s?

        For the OP, that’s what I got out of mine and I promptly dumped it for a 318.
         
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        • Slantsix64

          Slantsix64 Well-Known Member

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          If it's an auto more like 16-18mpg a four speed more like 19-22 mpg. If you do more street driving 3.23 would be better if more highway 2.76 or you can go in between 2.93. More timing. Try 6-8 instead of 0.
           
        • Slantsix64

          Slantsix64 Well-Known Member

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          79 horses is a little low you should be at around 90hp, mess with timing and a distributor recurve bigger exhaust 2.25 would be the cheapest way to gain power and mileage.
           
        • Slantsix64

          Slantsix64 Well-Known Member

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          • flingdingo

            flingdingo Well-Known Member

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            The only time ive recorded gas mileage in my wagon is on trips. It gets about 18 mpg, and that's loaded down with a family of five and a week's worth of luggage in the back. As others said, go through your tune-up carefully, you'll probably find some missing hp and mpg in there.
             
          • toolmanmike

            toolmanmike FABO Staff Staff Member FABO Gold Member

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            The guys getting mid 30's aren't running a 4:10 gear.
             
          • rumblefish360

            rumblefish360 So close, yet so far away FABO Gold Member

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            What’s the OD ratio of the manual trans?
             
          • Rainy Day Auto

            Rainy Day Auto Princess Valiant FABO Gold Member

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            I daily drive a 68 dart with a slant six and an 833 od. I get 14 in town and 17 on the highway.

            I have electronic ignition on it and I could probably get better mileage if I was easier on it but I tend to be a lead foot and don't let a lot of people pass me.
             
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            • toolmanmike

              toolmanmike FABO Staff Staff Member FABO Gold Member

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              Whatever the Mopar o/d 's are.
               
            • Rainy Day Auto

              Rainy Day Auto Princess Valiant FABO Gold Member

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              3.09, 1.67, 1.00, and 0.73
               
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              • rumblefish360

                rumblefish360 So close, yet so far away FABO Gold Member

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                Thanks Rani.
                .73 X 4.10 = 2.99 final drive ratio.
                 
              • Rainy Day Auto

                Rainy Day Auto Princess Valiant FABO Gold Member

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                I put 3.23 in my car . the original 2.76 was kinda mushy on the take off, felt like you might give it a push to get going. The 3.23 are much more responsive but still very mild in the big picture.

                In town I never hit the OD gear, mostly run a gear down in traffic for control and throttle response.

                I would like to get some deeper gears later, because I don't care about gas mileage. Gas is cheap in America, even at 3 bucks a gallon, that's a steal.
                 
              • RustyRatRod

                RustyRatRod Weenie idiot loser. FABO Gold Member

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                In their wet dreams, that's where. No slant ever got that. If somebody thinks they got that in a slant, they either don't know how to measure mileage, or flat out lied. Ain't happenin.
                 
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                • RustyRatRod

                  RustyRatRod Weenie idiot loser. FABO Gold Member

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                  .73 in cars and .71 in trucks.
                   
                • lilnoah

                  lilnoah Supercharged slant six with 4 speed and 8.25

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                  My slant ride gets 14-18 mpg with 6# of boost and a833 and 2.46
                   
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                  • rumblefish360

                    rumblefish360 So close, yet so far away FABO Gold Member

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                    Thanks brother!
                    Ahhhh man! Remember when I first had the /6 in that ‘74 Duster and I was getting 19-ish in the Hwy? The old timers were beating me up on not knowing how to tune the engine and get what they were claiming, 30, 32, 35+ mog’s.

                    I never believed that shit but! When they were new and the big 3 had mpg Competitions ......
                     
                  • rumblefish360

                    rumblefish360 So close, yet so far away FABO Gold Member

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                    On the cheap, double check spark plug condition, move timing to 10*’s or better, check all filters & change fluids. Inflate tires correctly. Check front end alignment. These are all players in the big picture.
                    Good luck.

                    The above mentions of parts to upgrade are IMO good but are they an OK extra expenditure for you?
                    I myself would go that route after getting the most out of the stock system you have now.

                    A better exhaust, which can still be quite, just larger pipe diameters. 2-1/4 should do it just fine.

                    Upgraded ignition as well.
                     
                  • RustyRatRod

                    RustyRatRod Weenie idiot loser. FABO Gold Member

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                    Yeah the "light" cars may have gotten "around" 30, but that was the best anything ever did with a slant. I always wanted to do one up using a Weber staged two barrel and Super Six intake. With the right exhaust and an OD transmission, that could get good. The primary barrel is small and would lend itself to really good mileage. .........but my problem is, I NEVER build for mileage. IF what I build gets decent mileage, it's a simple after thought. lol
                     
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